1. Field of the Invention
The present invention relates to a sheet-feeding device for feeding sheets one by one.
2. Description of the Related Art
In image-forming apparatuses, such as copy machines and printers, sheets of paper which can be fed continuously are normally limited to sheets of high-quality paper or normal paper produced by designated manufacturers. These kinds of paper have non-smooth surfaces and are highly permeable to air (easy for air to pass through), and air can easily flow between the sheets. Therefore, when the sheets stored in a stacked state are taken out one by one, double feeding, in which two or more sheets adhere to each other and are fed together, does not easily occur.
On the other hand, in accordance with the diversification of recording paper and growing need for color printing, there is an increasing demand to form images on sheets with smooth surfaces, such as cardboard sheets, overhead projector (OHP) sheets, sheets of tracing paper, and sheets of art paper and coated paper having surfaces coated to increase whiteness or glossiness. However, since the OHP sheets, the sheets of tracing paper, and the sheets of art paper and coated paper have smooth surfaces and are not very permeable to air (difficult for air to pass through), they easily stick to each other, particularly when they are stacked in high-humidity conditions. Therefore, there is a problem that double feeding or misfeeding easily occurs when a friction method, which is commonly used in known copy machines and printers, is used for separating the sheets.
Accordingly, methods for preventing sheets with low air-permeability from sticking to each other have been suggested. For example, U.S. Pat. No. 6,015,144 discloses a structure in which air is blown against a stack of sheets from the side to reduce the adhesion force between the sheets having smooth surfaces. In addition, Japanese Patent Laid-Open No. 6-32473 discloses a structure in which air is heated by a dehumidifier heater disposed in a lower section and is blown along the top surface of the uppermost sheet on a paper tray or along the side of a stack of sheets by an air-exhaust ventilator to reduce the adhesion force between the sheets having smooth surfaces. In addition, Japanese Patent Laid-Open No. 2001-48366 discloses a structure including an air heater for maintaining the humidity of air to be blown against sheets within a predetermined level by heating the air depending on the temperature or humidity of external air and a pre-heating temperature detector for detecting the temperature of the air before being heated, so that the conveyability of the sheets is improved and the quality of the output sheets is maintained.
In known sheet-feeding devices, when a storage unit is opened for refilling it with paper or for other reasons, electricity to the heater is normally shut off in order to save power. Accordingly, the temperature of the heater decreases when the storage unit is opened, and a long time is required for the temperature of the heater to reach a set temperature after the storage unit is closed. Accordingly, the downtime of the image-forming apparatus required when the storage unit is refilled with paper is long. The downtime of the image-forming apparatus may be reduced by not shutting off the electricity to the heater when the storage unit is opened. In such a case, however, since the temperature decreases when the storage unit is opened, there is a risk that misdetection of a low-temperature error, that is, an abnormality in which the temperature of the heater does not reach a predetermined temperature, will occur.